The present invention starts out from a method for the preparation of one-phase, intermetallic phases, which melt incongruously within a temperature range of 900.degree. to 2,000.degree. K. and have a homogeneity range of .ltoreq.10 atom percent at room temperature. These phases are prepared by the calciothermal reduction of a finely divided, homogeneous mixture of the alloying components, of which at least one is present in the form of an oxide, subsequent diffusion of the alloying components and separation of the calcium oxide formed and of any excess calcium. This method is the object of the German Offenlegungsschrift . ... ... (unpublished German patent application P 42 04 173.2 of 2-13-1992) and has the combination of the following characteristics:
a) Adjusting the exothermic energy of the calciothermal reduction by the oxide content of the reaction mixture, which corresponds in its composition to the desired single phase alloy, in such a manner that the temperature condition T.sub.M &gt;T.sub.R .gtoreq.0.9 T.sub.M (in degrees Kelvin) is fulfilled, and T.sub.M being the melting temperature of the intermetallic phase and T.sub.R being the reaction temperature,
b) using a reaction mixture, the components of which, with the exception of calcium, have an average particle size of .ltoreq.75 .mu.m,
c) tempering the reaction product at the end of the exothermal reaction at a temperature, which is at least 0.7 times the melting temperature T.sub.M of the desired single phase alloy, measured in .degree. K., but in less than the melting temperature T.sub.M, during a period of time sufficient for the diffusion of the components.
This method is the starting point of the present invention, which is concerned with the technical problem of producing alloys of the Se.sub.2 Fe.sub.17-x TM.sub.x N.sub.y type (x=0 to 10, y=&gt;0 to 5), which preferably have a magnetic anisotropy in the direction of the c axis, from alloys of the type SE.sub.2 Fe.sub.17 (SE stands for a rare earth metal, including capital Y, or a mixture of these metals, and TM stands for Co, Ni, Cu, Zr, Ga, Hf, Ta, Nb, Ti, Si, A1, V, Mo, Cr, Zn or Sn or a mixture of these metals), based essentially on the single phase, incongruently melting intermetallic phase SE.sub.2 Fe.sub.17 obtainable from the aforementioned method.
Alloys of the Se.sub.2 Fe.sub.17-x TM.sub.x N.sub.y are known from the state of the art. Compared to the base alloy, they have the advantage of a higher Curie temperature. For example, the Curie temperature of the Sm.sub.2 Fe.sub.17 alloy is 130.degree. C., while the Curie temperature of the Sm.sub.2 Fe.sub.17 N.sub.y alloy, with y=3, is 470.degree. C.
Nitrides of the composition RE.sub.60 Fe.sub.(100-.alpha.-.beta.-.gamma.)N.beta.H.sub..gamma. are described in the German Offenlegungsschrift 0 369 097, in which the subscripts have the following values in atom percent: .alpha.=5 to 20, .beta.=5 to 30 and .gamma.=0.01 to 10.
The European publication 0 453 270 is also concerned with the hard magnetic properties of nitrides of the aforementioned composition.
For the preparation of for example, Sm.sub.2 Fe.sub.17 N.sub.y, an, as far as possible, single phase Sm.sub.2 Fe.sub.17 is required as preliminary product, in order to convert this product subsequently by nitriding into the desired nitride. Frequently, the Sm.sub.2 Fe.sub.17 is alloyed by different elements, in order to improve the nitriding behavior or, for example, the magnetic properties of the subsequently produced nitride. For example, by the addition of Nb to melt metallurgically produced Sm.sub.2 Fe.sub.17, the soft magnetic .alpha.-Fe, which occurs unavoidably, can be bonded in the form of an intermetallic Laves phase of the composition NbFe.sub.2 (A.E. Platts, I.R. Harris, J.R.D. Coye, Journal of Alloys and Compounds, 185, 251 (1992)). In this case, after the nitriding, Sm.sub.2 Fe.sub.17 N.sub.y is formed, from a multiphase pre-alloy, which is based, however, essentially on the intermetallic phase Sm.sub.2 Fe.sub.17.
Sm.sub.2 Fe.sub.17 is an incongruently melting intermetallic phase with a melting point of 1,280.degree. C. At the melting point of the Sm.sub.2 Fe.sub.17, the following thermodynamic equilibrium exists:
Sm.sub.2 Fe.sub.17 =.alpha.-Fe+Sm-rich melt, as described in J. Less-Common Metals, 25, 131 (1971).
While examining the magnetic properties of Sm.sub.2 Fe.sub.17 Ny.sub.y, it was observed by means of a hysteresis curve that residues of .alpha.-Fe are still present. The presence of these residues decrease the coercive force of the desired nitride and should therefore be prevented as far as possible.
In order to be able to compensate for the unavoidable precipitation of .alpha.-Fe during the melt metallurgical preparation of Sm.sub.2 Fe.sub.17, a time-consuming diffusion annealing of up to 2 hours at 1,000.degree. C. is required.
The adjustment of a nanocrystalline structure represents a second possibility for obtaining Sm.sub.2 Fe.sub.17. Schnitzke et al. (K. Schnitzke, L. Schultz, J. Wecker, M. Katter, submitted to Appl. Phys. Lett.) at first prepared a nanocrystalline Sm.sub.2 Fe.sub.17 by mechanically alloying the element powder. Radiographic analysis of the powder revealed an amorphous phase and crystalline e-Fe. The intermetallic Sm.sub.2 Fe.sub.17 phase is formed only during a subsequent heat treatment and is then converted by nitriding to the Sm.sub.2 Fe.sub.17 N.sub.y compound.
It is a disadvantage of this preparative method that only isotropic Sm.sub.2 Fe.sub.17 N.sub.y alloys can be produced.
The thermodynamically preferred reactions during the nitriding of Sm.sub.2 Fe.sub.17 are:
Sm.sub.2 Fe.sub.17 +N.sub.2 2SmN+17Fe Sm.sub.2 Fe.sub.17 +y/2N.sub.2 Sm.sub.2 Fe.sub.17 N.sub.y 2SmN+(y-2)/2N.sub.2 17Fe
These reactions are affected by temperature and duration of nitriding. With increasing temperature and/or length of time of nitriding, an increased formation of SmN and .alpha.-Fe is observed. The nitriding can be accelerated by decreasing particle size of the Sm.sub.2 Fe.sub.17 because of a shorter diffusion path.
Consequently, at constant temperature and nitriding atmosphere, the parameters of particle size and length of time and temperature of the nitriding must be coordinated with one another in preliminary experiments, in order to be able to produce the required nitride in the desired purity.
It was found that the preparation of the nitride of the Se.sub.2 Fe.sub.17-x TM.sub.x N.sub.y type can be ensured in a manner that is simple from a chemical engineering point of view, with improved economic efficiency and in the required quality, by combining the calciothermal co-reduction with the nitriding in one process step and maintaining selected process conditions.